The invention relates to an apparatus for distributing a liquid across a packed bed of a vapor-liquid contact column.
Packed columns for vapor-liquid contact are used in various operations, such as distillation, fractionation, absorption, stripping and heat exchange. The columns are usually filled with packing material that is randomly-oriented. However, on occasion the packing material is carefully positioned. The columns are usually used in operations involving counter-current flow of the vapor and liquids in the column, but are suitable for uses involving co-current flows as well.
Different applications indicate the use of differing packing arrangements and differing means of distributing the liquid onto the packing. For example, operations requiring less than thirty linear feet of packing and no intermediate feed or takeoff streams generally require a column having only a single bed of packing therein, and one device for distributing the liquid onto the packing. In these columns the device is mounted above the packed bed and is known as a liquid "distributor." Other operations require a column having more than one packed bed. In such an instance the column requires additional distributing devices which are positioned between the packed sections. The additional distributing devices are known as liquid "redistributors."
There are two general types of conventional liquid distributors on the market today. The first is the orifice type, which is characterized by head-driven flow through orifices, tubes, or nozzles which are located in the bottom of a pan, trough, or pipe. Examples of this type include distributors having orifice plates, perforated pipes, or spray nozzles.
Other conventional liquid distributors are of the overflow weir type. These distributors are characterized by flow-through weirs cut into the tops or sides of tubes set in the bottom of a pan, or weirs cut into the wall of troughs laid out in a ladder pattern. The weir design is known for its minimal tendency to foul owing to the self-cleaning nature of the weirs. In a typical vapor-liquid contact operation, the liquid distributor of this type provides for the liquid to enter the tubes or troughs. As the liquid overflows the weirs it falls onto the packing in the column. Vapor entering the bottom of the column rises up through the packing, where it contacts the downflowing liquid.
However, a number of problems are encountered in the use of these known distributing devices. First of all, the function of the distributors and redistributors in vapor-liquid contact operations is to ensure uniform liquid distribution across the cross-section of the tower. Uneven liquid distribution is referred to as "maldistribution." Maldistribution results in a reduced transfer of mass and/or heat between the vapor and liquid phases, which consequently can result in poor separation of components in the column. It can have other detrimental effects as well, such as material decomposition in processes which utilize temperature-sensitive liquids as reactants.
Maldistribution is a particularly common problem when the conventionally used distributors are used in packed columns having large diameters, that is, greater than about eighteen inches, due to the sensitivity to leveling. This problem is particularly acute in operations such as vacuum fractionation, where the liquid loading per cross-sectional area of the column is low. In the larger columns the longer span of the weir-type or orifice type system makes it much more difficult to maintain a level position, particularly since most distributor systems are not precisely level even when first installed. Later, as the column shifts, the distributors become even further out of level. In a system which is not level, the liquid collects at the lower end of the trough or tube and the higher end is thus "starved" for liquid.
A second problem encountered is that of fouling. While this is generally not a problem with the weir-type devices, it can present major difficulties in an orifice-type system. This is particularly so in processes such as vacuum fractionation, and other operations which require a low flow rate of liquid from the distributor to the packing, because for these applications the liquid must drain through very small orifices. Solids in the liquid often plug these drain orifices, requiring time-consuming and expensive servicing, and resulting in poor operation while plugged. Plugging problems are also encountered in designing systems that are less sensitive to leveling. For this also it is necessary to use very small orifices--often less than 1/4 inch in diameter--in order to ensure a sufficiently high liquid head, which in turn operates to reduce maldistribution problems.
Finally, there is often a problem in efficient use of space. High turndown/turnup ratios, i.e., large differences between the minimum and maximum liquid flow rates, necessitate additional liquid head. This is shown by the orifice equation: EQU q=CA(2gh).sup.0.5
In the above equation q is the fluid flow rate in cubic feet per second, C is the dimensionless orifice coefficient for the particular orifice being used, A is the flow area of the orifice in square feet, g is the acceleration due to gravity (32.2 feet per second per second), and h is the height of fluid held up over the orifice in feet of flowing fluid, i.e., the head. From this equation it is seen that as the flow rate changes, the head must also change. This change is proportional: The head required at maximum flow is equal to the head at minimum flow multiplied by the square of the turndown/turnup ratio. Thus, for example, if a 2 inch head is to be maintained at minimum flow and there is a turndown/turnup ratio of 3:1, the head required at maximum flow would be 18 inches. However, because of this relationship serious problems arise particularly as to the redistributors, since in a packed column even relatively small reductions in packed space can significantly reduce column efficiency, and it is not always possible to simply enlarge the column.
Solutions or improvements to some of the above-noted problems have been provided by the invention described in U.S. Pat. No. 4,472,325, entitled "Liquid Distributor Apparatus for a Vapor-Liquid Contact Column." That invention consists of a distributor unit and a parting manifold which operate together to maximize uniform distribution, even at very low flows, while reducing fouling problems. The distributor unit is generally a plate with upstanding walls positioned crosswise and lengthwise to define separate, open-top compartments. Installed in each compartment are a number of drip tubes which extend above and below the plate. Gas riser openings are positioned between certain compartments, and the distributor unit is mounted inside the column above the packed bed. The parting manifold is mounted above the distributor unit, and includes a header circuit, crosswise end conduits, a crosswise lateral conduit, and a vertical standpipe conduit. The end conduits and lateral conduit have closed ends and each lateral includes outlet ports therein. The downflowing liquid enters the manifold though the standpipe conduit, drops through the outlet ports into each compartment, and flows downwardly through the drip tubes onto the packing.
That invention solves or reduces the first problem above, namely that of leveling and resultant "starvation" of one portion of the column. However, it still has a number of problems. Plugging of the orifices of the parting manifold can still occur, requiring expensive cleaning and servicing. The servicing is complicated by the fact that the parting manifold generally is hard-piped, and must be dismantled for cleaning and then reinstalled. Furthermore, that invention does not address the problems resulting from high turndown ratios. It is still necessary to maintain a substantially increased liquid headtto accomodate the high turndown ratio, and this in turn reduces the efficiency of the column and/or necessitates substantial design modifications.
In view of the unsatisfactory nature of prior liquid distributors, it would be desirable to have a distributor which would better address the problems of high turndown ratios, difficult servicing, fouling, and maldistribution.